Drilling fluid is liquid circulating throughout the borehole during rotary drilling or workover operation of aging borehole, and therefore have the functions below:
(1) drilling fluid transports cuttings from the bottom of well to the surface of the ground;
(2) drilling fluid balances pressure in borehole to prevent formation fluid from blowing out;
(3) drilling fluid aids to form a thin and tough mud wall protecting the borehole wall, and thereby preventing the subterranean formation damage;
(4) drilling fluid keeps drilling rigs cool and lubricates them; and
(5) drilling fluid carries cuttings without sinking therein even if the drilling fluid is stopped circulation.
Drilling fluid is classified into water-based drilling fluid and oil-based drilling fluid. The drilling mud consisting mostly of water-bentonite suspension, and further containing additives such as dispersing agent, weighting material, polymers, salts, and lubricant for adjusting the drilling mud to the borehole condition, is commonly used.
Bentonite primarily contains clay mineral called as montmorillonite, and also contains another minerals such as feldspar and quartz. Such bentonite increases viscosity of the drilling fluid, and supports to transport the cuttings to the ground. Also, the bentonite aids to form mud wall on the surface of the borehole wall during drilling operation, and thereby minimizing subterranean formation damage. This function of the bentonite is explained as follows. Since the pressure of drilling fluid in the borehole is usually settled higher than the formation pressure, clay mineral (e.g. bentonite) contained in the drilling fluid invades pores of the formation in the borehole while drilling, and as a result, the mud wall is formed on the surface of the borehole wall. The resulting mud wall aids to prevent the subterranean formation damage during drilling operation, and reduce the friction against the drill string, and so on.
However, in the case of a simple drilling mud obtained by mixing bentonite with water and adding dispersing agent and weighting material to the mixture according to the condition of the borehole, such drilling mud may be used only for drilling shallow stratum or the like due to disadvantages of insufficient resistance to damage, salt- or cement-sensitive property, and unstability to temperatures. In order to provide a drilling fluid having the above-mentioned functions (1)-(5), even when used for a large scale construction or underground continuous wall construction, addition of various additives is required.
Examples of typical additives include a salt containing potassium ion and the like, and a polymer such as acrylate-based polymer, carboxymethyl cellulose, and polysaccharide.
It is known that potassium ion is superior to other cations capable of electrically adsorbing to montmorillonite, with respect to suppression of swelling of montmorillonite. It is also known that the above-mentioned polymers enable to enhance a resistance of bentonite drilling mud to cement contamination, and enable to inhibit excessive viscosity increase of the drilling mud by acting as a dispersing agent of the drilling mud.
Besides the above-listed polymers, a modified polyvinyl alcohol (called as “modified PVA”) is also known as a polymer added to drilling fluid. For example, JP S57-23671A suggests the use of carboxylic acid-modified PVA, and JP H08-85710A suggests the use of sulfonic acid-modified PVA having a specific chemical structure.
JP S57-23671A discloses that the use of carboxylic acid-modified PVA can form a thinner and stable membrane comparing to the use of conventional stabilizers (e.g. carboxymethyl cellulose, non-modified PVA, and sulfonic acid-modified PVA). JP H08-85710A discloses that the specific sulfonic acid-modified PVA is superior to acrylate-based polymer with respect to dispersibility of the drilling mud.
The bentonite drilling mud containing a modified PVA is supposed to be excellent in dispersibility and formability of mud wall, comparing with a common bentonite drilling mud containing acrylate-based polymer, carboxymethyl cellulose, or polysaccharide, or the like.
Montmorillonite as a main component of bentonite has a structure of layered crystalline, and therefore swells by invasion of water between layers thereof. Such hydration swelling of montmorillonite may cause to damage the membrane formed on the borehole wall. Further, destruction of layered structure caused from the hydration swelling of bentonite varies a viscosity of the drilling fluid, and therefore the drilling fluid may not ensure the sufficient function of transporting drill cuttings upward to the ground.
US2007/0129258A discloses that the use of cation-modified PVA-based resin together with shale hydration inhibitor agent can prevent hydration swelling of shale clay to control a rheological property.
However, the cation-modified PVA-based resin used in US2007/0129258A, does not have a satisfied effect of preventing collapse of the membrane on the wall caused by swelling of bentonite. Accordingly, a further excellent drilling fluid additive is desired.